Wafer support system

ABSTRACT

A wafer support system comprising a segmented susceptor having top and bottom sections and gas flow passages therethrough. A plurality of spacers projecting from a recess formed in the top section of the susceptor support a wafer in spaced relationship with respect to the recess. A sweep gas is introduced to the bottom section of the segmented susceptor and travels through the gas flow passages to exit in at least one circular array of outlets in the recess and underneath the spaced wafer. The sweep gas travels radially outward between the susceptor and wafer to prevent back-side contamination of the wafer. The gas is delivered through a hollow drive shaft and into a multi-armed susceptor support underneath the susceptor. The support arms conduct the sweep gas from the drive shaft to the gas passages in the segmented susceptor. The gas passages are arranged to heat the sweep gas prior to delivery underneath the wafer. Short purge channels may be provided to deliver some of the sweep gas to regions surrounding the spacers to cause a continuous flow of protective purge gas around the spacers. A common bottom section may cooperate with a plurality of different top sections to form segmented susceptors suitable for supporting various sized wafers.

RELATED APPLICATIONS

This application claims the priority benefit of Provisional applicationNo. 60/039,850 filed Mar. 5, 1997 and is a continuation-in-part ofapplication Ser. No. 08/788,817 filed Jan. 23, 1997, which is acontinuation-in-part of application Ser. No. 08/706,069 filed Aug. 30,1996, (now abandoned) which claims the priority benefit of ProvisionalApplication No. 60/003,132, filed Sep. 1, 1995.

FIELD OF THE INVENTION

The present invention relates to supports for wafers in semiconductorprocessing chambers and, more particularly, to a system for supporting awafer above a susceptor within a chemical vapor deposition chamber.

BACKGROUND OF THE INVENTION

High-temperature ovens, or reactors, are used to process semiconductorwafers from which integrated circuits are made for the electronicsindustry. A circular wafer or substrate, typically made of silicon, isplaced on a wafer support called a susceptor. Both the wafer andsusceptor are enclosed in a quartz chamber and heated to hightemperatures, such as 600° C. (1112° F.) or higher, frequently by aplurality of radiant lamps placed around the quartz chamber. A reactantgas is passed over the heated wafer, causing the chemical vapordeposition (CVD) of a thin layer of the reactant material on the wafer.Through subsequent processes in other equipment, these layers are madeinto integrated circuits, with a single layer producing from tens tothousands of integrated circuits, depending on the size of the wafer andthe complexity of the circuits.

If the deposited layer has the same crystallographic structure as theunderlying silicon wafer, it is called an epitaxial layer. This is alsosometimes called a monocrystalline layer because it has only one crystalstructure.

Various CVD process parameters must be carefully controlled to ensurethe high quality of the resulting semiconductor. One such criticalparameter is the temperature of the wafer during the processing. Thedeposition gas reacts at particular temperatures and deposits on thewafer. If the temperature varies greatly across the surface of thewafer, uneven deposition of the reactant gas occurs.

In certain batch processors (i.e., CVD reactors which process more thanone wafer at a time) wafers are placed on a relatively large-masssusceptor made of graphite or other heat-absorbing material to help thetemperature of the wafers remain uniform. In this context, a"large-mass" susceptor is one which has a large thermal mass relative tothe wafer. Mass is equal to the density times volume. The thermal massis equal to mass times specific heat capacitance.

One example of a large-mass susceptor is shown in U.S. Pat. No.4,496,609 issued to McNeilly, which discloses a CVD process wherein thewafers are placed directly on a relatively large-mass, slab-likesusceptor and maintained in intimate contact to permit a transfer ofheat therebetween. The graphite susceptor supposedly acts as a thermal"flywheel" which transfers heat to the wafer to maintain its temperatureuniform and relatively constant. The goal is to reduce transienttemperature variations around the wafer that would occur without the"flywheel" effect of the susceptor.

In recent years, single-wafer processing of larger diameter wafers hasgrown for a variety of reasons including its greater precision asopposed to processing batches of wafers at the same time. Althoughsingle-wafer processing by itself provides advantages over batchprocessing, control of process parameters and throughput remainscritical. In systems in which the wafer is supported in intimate contactwith a large-mass, slab-like susceptor, the necessity of maintaininguniform susceptor temperature during heat-up and cool-down cycleslimited the rate at which the temperature could be changed. For example,in order to maintain temperature uniformity across the susceptor, thepower input to the edges of the susceptor had to be significantlygreater than the power input to the center due to the edge effects.

Another significant problem faced when attempting to obtain high-qualityCVD films is particulate contamination. One troublesome source ofparticulates in the CVD of metals and other conductors is the film thatforms on the back side of the wafer under certain conditions. Forexample, if the wafer back side is unprotected or inadequately protectedduring deposition, a partial coating of the CVD material forms thereon.This partial coating tends to peel and flake easily for some types ofmaterials, introducing particulates into the chamber during depositionand subsequent handling steps. One example of protecting the back sideof a wafer during processing is given in U.S. Pat. No. 5,238,499 to vande Ven, et al. In this patent an inert gas is introduced through acircular groove in the peripheral region of a support platen. In U.S.Pat. No. 5,356,476 to Foster, et al., a semiconductor wafer processingapparatus is shown, including a plurality of ducts for introducinghelium or hydrogen around the perimeter of a wafer to prevent flow ofreactant gases downwardly into a gap between the perimeter of the waferand a wafer support lip. The aforementioned devices, however, share thefeature of rather large wafer support platens, characterized by theaforementioned detrimental high thermal mass.

Presently, there is a need for an improved wafer support system whileensuring temperature uniformity across the wafer surface.

SUMMARY OF THE INVENTION

The present invention embodies a susceptor which supports a wafer spacedtherefrom and effectively decouples conductive heat transfer between thetwo elements. The wafer is supported on one or more spacers in a recesspreferably in an upper surface of the susceptor, the top plane of thewafers preferably being approximately level with an outer ledge of thesusceptor. In one arrangement, spacer pins are utilized, and in anothera single spacer ring is used. The susceptor preferably includes aplurality of interior passages opening into the recess at a plurality ofsmall sweep gas holes. A sweep gas flows through the susceptor and outthe holes and protects the back side of the wafer from deposition gasand particulate contamination. The sweep gas is heated as it flowsthrough the susceptor so as not to cause localized cooling of the waferand possible areas of slip.

In one embodiment, the susceptor is formed by top and bottom matingsections and the internal passages are formed by grooves in one of thejuxtaposed surfaces of the two sections. Desirably, a multi-armed membersupports and rotates the susceptor, the member preferably beingsubstantially transparent to radiant energy. The arms of the supportmember are preferably hollow and deliver sweep gas to the lower surfaceof the susceptor at apertures in communication with the internalpassages. Some of the sweep gas may be diverted to exit the susceptorproximate the spacer pins to provide sweep gas protection therearound atall times.

In another aspect of the invention the spacer ring mentioned is locatedto be positioned beneath the periphery of the wafer and serves to reducethe size of the sweep gas outlet from beneath the wafer and to blockdeposition gas from flowing to the wafer backside. The ring isconfigured to support the wafer in one arrangement. Preferably, the ringand the susceptor are configured to form sweep gas outlet passages. Asanother embodiment, the ring is spaced slightly from the wafer toprovide a thin annular outlet for the sweep gas, and the wafer issupported by pins.

In one aspect, the invention provides a susceptor to be positioned in ahigh temperature processing chamber for supporting a wafer to beprocessed. The susceptor includes a thin, substantially disc shapedlower section and a thin, substantially disc shaped upper section havinga lower surface in engagement with an upper surface of said lowersection. One of the sections has an outer diameter larger than that ofthe other section, the larger section having a recess in which the othersection is positioned. One or more gas channels are defined by theengaging surfaces of the sections. The susceptor includes one or moregas inlets in the lower section opening to its lower surface and thechannels. One or more gas outlets in the upper section open to the uppersurface of the upper section in an area beneath that in which a wafer tobe processed is to be positioned. The mating recess is preferably formedin a lower surface of the upper section. In one form, the channels areformed by grooves in the upper surface of the lower section with thegrooves being closed by the lower surface of the upper section. Thereare preferably three of the inlets each opening to the channels, thechannels being interconnected to allow gas flow throughout.

In accordance with another aspect, the invention provides an apparatusfor chemical vapor deposition on a semiconductor wafer comprising adeposition chamber having a process gas inlet for injecting processgases into the chamber. A single susceptor is provided in the chamber. Asupport for the susceptor includes a central shaft positioned below thesusceptor axis and a plurality of support arms extending radially andupwardly from the shaft with the arms having upper ends adapted toengage the lower surface and support the susceptor. One or more of thearms are tubular and in registry with inlets in the susceptor so thatgas may be conducted through the tubular arms into the inlets.

The present invention also provides a method of supporting asemiconductor wafer in a processing chamber and conducting gas flowbeneath the wafer. The method comprises the steps of positioning thewafer on a plurality of spacers protruding upwardly from an uppersurface of the susceptor to support the wafer and form a gap between thewafer and the upper surface of the susceptor. The susceptor is supportedon a plurality of arms having upper ends engaging a lower surface of thesusceptor. Gas flows through one or more of the arms into passages inthe susceptor which open to the gap. The gas is allowed to flowoutwardly beyond the periphery of the wafer. Desirably, the spacers arepositioned in apertures in the susceptor, and some of the gas flows fromthe arms through the susceptor passages and into the gap via theapertures surrounding the spacers.

In another aspect of the invention, an apparatus for supporting wafersin a semiconductor processing environment includes a lower section and aplurality of disk-shaped upper sections each adapted to registerconcentrically with the lower section. The upper sections each have ashallow wafer recess sized differently than the other upper sections toenable selection of the upper section depending on the size of wafer tobe processed. The apparatus preferably includes at least two uppersections for processing wafers having diameters greater than 100 mm.

In a preferred form of the invention, a rotatable susceptor ispositioned generally horizontally in a processing chamber and one ormore spacers extend above the susceptors to support a single waferspaced from the susceptor. A temperature compensation ring surrounds butis slightly spaced from the susceptor and has a generally rectangularexterior shape. The chamber has at least one process gas inlet and atleast one gas outlet for flowing deposition and carrier gas across theupper surface of the wafer, and the chamber has a generally rectangularcross-section generally perpendicular to the gas flow across the waferand the rectangular ring. An inlet section of the chamber is verticallyshort and the susceptor and the ring are positioned adjacent the inletsection with the upper surface of the ring and the susceptor beinggenerally in the plane of the lower wall of the inlet section. The ringand the susceptor, together with a wafer mounted on the spacers areheated very uniformly by upper and lower heat sources. With thisarrangement, the gas has a generally uniform flow across the width ofthe chamber since deposition occurs on both the heated ring and thewafer. As a result, carrier gas flow is advantageously reduced from thatneeded with a circular susceptor and a circular temperature compensationring wherein it is usually necessary to have increased process gas flowacross the center of the wafer and reduced flow across the edges of thewafer in order to obtain uniform deposition on the wafer. The reducedcarrier gas flow is particularly desirable because of the reducedcooling effect on the thermally sensitive wafer spaced from thesusceptor. It is also desirable that the upper and lower heat sourceshave a generally rectangular heat pattern that coincides with the shapeof the exterior of the rectangular ring so that the heat is principallydirected to the area defined by the ring exterior.

In another aspect of the invention the system is provided with thecapability to modify the ratio of heat provided by upper and lower heatrecesses during the processing of a wafer, so as to promote rapiduniform heating.

With the wafer no longer in contact with the susceptor, the wafertemperature can be maintained uniform even where the susceptorexperiences temperature nonuniformities during heat-up and cool-down. Inthis manner, heat-up and cool-down times can possibly be reduced.Process throughput is thereby increased, as desired. Another aspect ofthe invention allows for the processing of wafers without the creationof haze or other undesirable effects on the underside of the wafer. Thisimprovement, provided by removing the wafer from contact with thesusceptor and bathing its underside with a gas, e.g. hydrogen, isparticularly important where doublesided polished wafers are beingprocessed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view along the longer of two horizontal axesthrough a reactor chamber incorporating an improved wafer support systemof the present invention;

FIG. 2 is a cross-sectional view through one embodiment of a wafersupport system of the present invention;

FIG. 2a is a detailed view of one embodiment of a wafer spacer in theform of a pin;

FIG. 2b is a detailed view of an alternative wafer spacer in the form ofa sphere;

FIG. 2c is a view of an alternative wafer spacer configuration;

FIG. 3 is an exploded view of the wafer support system illustrated inFIG. 2;

FIG. 4 is a top plan view of an upper section of a segmented susceptorof the wafer support system taken along line 4--4 of FIG. 3;

FIG. 5 is a top plan view of a lower section of the segmented susceptortaken along line 5--5 of FIG. 3;

FIG. 6 is a top plan view of a susceptor support for use in the wafersupport system of the present invention, taken along line 6--6 of FIG.3;

FIG. 7 is a cross-sectional view of another wafer support systemaccording to the present invention;

FIG. 8 is a top plan view of a segmented susceptor for use in the wafersupport system of FIG. 7, taken along line 8--8;

FIG. 9 is a top plan view of an alternative upper section of a segmentedsusceptor having gas outlets distributed around concentric circles;

FIG. 10 is a top plan view of an alternative lower section of asegmented susceptor having multiple gas delivery grooves arranged inconcentric circles;

FIG. 11 is a top plan view of a preferred wafer support system of thepresent invention;

FIG. 12 is a top plan view of a first version of a top section of asegmented susceptor for use in the wafer support system of FIG. 11;

FIG. 13 is a top plan view of a bottom section of the segmentedsusceptor of the wafer support system of FIG. 11;

FIG. 14 is a cross-sectional view of a captured wafer spacer and purgechannel within the segmented susceptor, taken along line 14--14 of FIG.11;

FIG. 15 is a top plan view of a second version of the top section of thesegmented susceptor for use in the wafer support system of FIG. 11;

FIG. 16 is a top plan view of a third version of the top section of thesegmented susceptor for use in the wafer support system of FIG. 11;

FIG. 17 is a top plan view of a fourth version of the top section of thesegmented susceptor for use in the wafer support system of FIG. 11;

FIG. 18 is a cross-sectional view through another variation of a reactorchamber incorporating the wafer support system of the invention;

FIG. 19 is a top plan view of the chamber of FIG. 18; and

FIG. 20 is a graph showing changes in lamp power ratio during adeposition cycle.

FIG. 21A is a top-plan view of the upper segment of another variation ofsegmented susceptor.

FIG. 21B is a top-plan view of the lower segment of a susceptor whichmates with the upper segment shown in FIG. 21A, a portion of which isshown.

FIG. 21C is a cross-sectional view of the segments of 21A and Bassembled and supporting a wafer.

FIG. 21D is an enlarged cross-sectional view of one edge of the assemblyof FIG. 21C illustrating more clearly the location of a support pin inreference to a wafer having a notch in its periphery.

FIG. 21E is a view similar to that of FIG. 21D but illustrating a waferhaving an edge alignment flat.

FIG. 22A illustrates a top-plan view of the lower segment of anothersegmented susceptor design with a portion of an upper segmentsuperimposed thereon to illustrate the relationship between the two.

FIG. 22B is an enlarged cross-sectional view of a portion of the upperand lower segments of FIG. 22A assembled and supporting a wafer.

FIG. 23A is top-plan view of an upper segment of another susceptor witha wafer support ring mounted on the upper segment, and with a portion ofa lower segment shown.

FIG. 23B is an enlarged cross-sectional view illustrating therelationship between the wafer support ring of FIG. 23A and a wafer.

FIG. 23C is an enlarged fragmentary view illustrating the cross-sectionof the sweep gas passages in the support ring on FIGS. 23A and 23B.

FIG. 24 is a plan view of another embodiment of a spacer or blockerring.

FIG. 25 is a view taken on line 25--25 of FIG. 24.

FIG. 25A is a view on line 25A--25A of FIG. 25, with a fragmentary,broken line showing of a susceptor and a wafer.

FIG. 25B is a view on line 25B--25B of FIG. 25.

FIG. 25C is a view on line 25C--25C of FIG. 25.

FIG. 26 is a plan view of another embodiment of a blocker ring.

FIG. 27 is a view on line 27--27 of FIG. 26.

FIG. 27A is a view on line on line 27A--27A of FIG. 27, with afragmentary, broken line showing of a susceptor and a wafer.

FIG. 27B is a view on line 27B--27B of FIG. 27.

FIG. 27C is a cross-sectional view of a variation of the ring of FIG.27B.

FIG. 27D is an enlarged view of the area identified by circle 27D shownon FIG. 27B.

FIG. 28 is a cross-sectional view of an alternative blocker ringconfiguration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a reactor chamber 20 for processing semiconductorwafers, within which a wafer support system 22 of the present inventionis incorporated. Prior to discussing the details of the wafer supportsystem 22, the elements of the reaction chamber 20 will be described.The support system is suitable for many types of wafer processingsystems, another one being shown in FIGS. 18 and 19, and the discussionherein should not be limited to one particular type of reaction chamber.

The chamber 20 comprises a quartz tube defined by an upper wall 24, alower wall 26, an upstream flange 28, and a downstream flange 30.Although not shown in the figure, the walls have a concave inner surfaceand a convex outer surface which, when viewed from a lateralcross-section, has a lenticular shape; and lateral edges of the reactionchamber 20 include relatively thick side rails between which a chambersupport plate 32 is attached. FIG. 1 is a longitudinal cross-sectionalong a central vertical plane of the chamber 20 illustrating thevertical dimension of the lenticular shape; the side rails are thus notseen. Preferably, the chamber 20 is manufactured from quartz. Thechamber support plate 32 reinforces the chamber 20 during vacuumprocessing and extends between the side rails (not shown), preferablyalong the center line of the chamber 20. The support plate 32 includesan aperture 33 defining a void or opening 35 extending across thelateral dimension of the chamber 20 between the side rails. The aperture33 divides the support plate 32 into an upstream section extending fromthe upstream flange 28 to an upstream edge of the aperture, and adownstream section extending from a downstream of the aperture to thedownstream flange 30. The upstream section of the support plate 32 ispreferably shorter in the longitudinal direction than the downstreamsection.

An elongated tube 34 depends from a centrally located region of thelower wall 26. A drive shaft 36 extends through the tube 34 and into alower region 38 of the chamber 20. The lower region 38 is definedbetween the central chamber support plate 32 and the lower wall 26. Theupper end of the drive shaft 36 is tapered to fit within a recess of amulti-armed support or spider assembly 40 for rotating a segmentedsusceptor 42. The susceptor 42 supports a wafer 44, shown in phantom. Amotor (not shown) drives the shaft 36 to, in turn, rotate the wafersupport system 22 and wafer 44 thereon within the aperture 33. A gasinjector 46 introduces process gas, as indicated by arrow 48, into anupper region 50 of the chamber 20. The upper region 50 is definedbetween the upper wall 24 and the chamber support plate 32. The processgas passes over the top surface of the wafer 44 to deposit chemicalsthereon. The system typically includes a plurality of radiant heat lampsarrayed around the outside of the reaction chamber 20 for heating thewafer 44 and catalyzing the chemical deposition thereon. An upper bankof elongated heat lamps 51 is illustrated outside of the upper wall 24,and typically a lower bank of lamps arranged cross-wise to the upperbank is also utilized. Further, a concentrated array of lamps directedupward from underneath the susceptor 42 is often used.

A source of sweep gas 37 is schematically shown connected through a massflow controller 39 to the drive shaft 36. Gas flows into the spacewithin the hollow shaft 36 and is eventually directed upward through thesusceptor 42, as will be more fully described below. The fluid couplingallowing gas to the interior of the hollow, rotating shaft 36 is notshown, but may accomplished by a number of means, one of which is shownand described in U.S. Pat. No. 4,821,674, issued Apr. 18, 1989, herebyexpressly incorporated by reference.

A wafer is introduced to the reaction chamber 20 through a wafer entryport 47. The wafer is typically transported by a robot pick-up arm (notshown) which enters through the port 47 and extends over the wafersupport system 22 to deposit the wafer thereon. The CVD system thenseals the reaction chamber 20 and introduces deposition gas with acarrier gas such as hydrogen for depositing a layer of silicon or othermaterial on the wafer. After processing, a gate valve opens and therobot pick-up arm enters through the port 47 and retracts the wafer fromthe susceptor 42. Periodically, the reaction chamber 20 must beconditioned for subsequent processing. A typical sequence is theintroduction of an etch gas into the reaction chamber with the gatevalve closed to clean any particular deposition from the interior walls.After the etching, a silicon precursor is sometimes introduced into thechamber to provide a thin coat of silicon on the susceptor 42. Such acoating step is sometimes termed capping. After the etching and cappingsteps, the chamber is purged with hydrogen and heated for introductionof the next wafer.

The tube 34 is sized slightly larger than the drive shaft 36 to providespace therebetween through which purge gas 52 flows. The purge gasenters the lower region 38 of the reaction chamber 20 to help preventreactant gas from depositing in the lower region. In this respect, thepurge gas 52 creates a positive pressure below the wafer support system22, which helps prevent reactant gas from traveling around the sides ofthe segment susceptor 42 in the lower region 38. The purge gas is thenexhausted, as indicated with arrows 55, between the susceptor 42 andaperture 33 into the upper region 50 and then through an elongated slot60 in the downstream flange 30. This ensures that reactant gases do notmigrate into the lower region 38. The purge gas continues through anexhaust system 58. The reactant gas likewise passes through theelongated slot 60 in the downstream flange 30 to be vented through theexhaust system 58.

Preferably, a temperature compensation ring 62 surrounds the wafersupport system 22. The ring 62 fits in the opening 35 created by theaperture 33 in the support plate 32, and the wafer support system 22 andring substantially fill the opening and provide structure between thelower and upper chamber regions 38, 50. The susceptor 42 rotates withinthe ring 62 and is preferably spaced therefrom across a small annulargap of between 0.5 and 1.5 millimeters. The shape of the aperture 33 inthe support plate 32 surrounding the ring 62 can be made circular sothat the edges of the opening 35 are in close proximity to the ring.However, it has been found that a generally rectangular aperture 33 ispreferred. In this respect, the ring 62 may have a generally rectangularouter periphery, or a second structure may be utilized to fill the gapbetween the circular ring and the aperture 33. As will be described ingreater detail below, the susceptor 42 is preferably manufactured tohave a constant outer diameter to fit within the ring 62, andsurrounding aperture 33. Although the susceptor 42 has a constant outerdiameter, it will be seen that various configurations are provided forprocessing a number of different size wafers.

In a particularly advantageous embodiment, the temperature compensationring 62 comprises a two-part structure circular ring having a cavitytherein for receiving thermocouples 64. In the embodiment shown, thethermocouples 64 enter the chamber 20 through apertures formed in thedownstream flange 30 and extend underneath the support plate 32 into thetemperature compensation ring 62. The apertures in the quartz flange 30substantially prevent gas leakage around the thermocouples 64, althoughtypically no additional seal is used. There are preferably three suchthermocouples, one terminating at a leading edge 66, one terminating ata trailing edge 68, and one terminating at either of the lateral sidesof the ring 62. The thermocouples within the ring 62 surrounding thesegmented susceptor 42 provide good temperature information feedback foraccurate control of the radiant heating lamps. A plurality of bentfingers 70 attached to the support plate 32 support the ring 62 aroundthe periphery of the susceptor 42. In addition to the ring 62 andthermocouples therein, a central thermocouple 72 extends upward throughthe drive shaft 36, which is hollow, and through the spider assembly 40to terminate underneath the center of the susceptor 42. The centralthermocouple 72 thus provides an accurate gauge of the temperature nearthe center of the wafer 44. Because the temperature of a wafer changesquickly in the present system, it is desirable that the mass of thethermocouples be minimized to speed response time.

Referring to FIG. 2, a first embodiment of a wafer support system 22 isshown. Again, the system 22 generally comprises the segmented susceptor42 supported by arms 74 of the spider assembly 40. The arms 74 extendradially outward from a hub 76 and bend vertically upward atpredetermined radial distances to contact the underside of the susceptor42. The segmented susceptor 42 comprises an upper section 78 and a lowersection 80, both sections being generally planar disk-shaped elements.Both sections 78, 80 of the susceptor 42 are preferably machined out ofgraphite and fit closely together without additional fastening means toensure minimal gas leakage therebetween. A gap of less than 0.001 inchbetween the adjacent circular surfaces of the upper and lower sections78, 80 is acceptable for this purpose. A thin coating of silicon carbidemay be formed on one or both sections 78, 80. The thickness of thesusceptor 42 is preferably about 0.30 inches.

With reference to the exploded view of FIG. 3, the upper section 78generally comprises an outer ring 82 surrounding a thinner circularmiddle portion. The outer ring 82 comprises an upper rim or ledge 84 anda lower rim or skirt 86 which terminate at upper and lower shoulders orsteps 88, 90, respectively. The upper step 88 forms a transition betweenthe ledge 84 and a circular wafer-receiving recess 92. The lower step 90forms a transition between the skirt 86 and an annular recess 94 in theunderside of the upper section 78. The upper section 78 further includesa circular pattern of sweep gas outlets 96 symmetrically disposed aboutthe central axis of the upper section, and in the recess 92.

At spaced locations distributed around a circle concentric about theaxis of the susceptor 42, a plurality of counter-bored holes 98 areformed proximate the upper step 88. The counter-bored holes 98 include asmaller through hole opening to the circular recess 42 and a largercounterbore concentric with the smaller through hole and openingdownwardly to the annular recess 94. Each counter-bored hole 98 is sizedto receive a wafer support or spacer 100 which projects into thecircular recess 92. The wafer 44 rests on the spacers 100 above thefloor of the recess 92. In this respect, the recess 92 is sized toreceive a wafer therein so that the edge of the wafer is very close tothe step 88. The upper section 78 further includes a downwardlydepending central spindle 102 defining a radially inner border of theannular recess 94. A central thermocouple cavity 104 is defined in thespindle 102 for receiving a sensing end of the central thermocouple 72previously described.

With reference to FIGS. 3 and 5, the annular lower section 80 comprisesa central through bore 106 sized to fit around the downwardly dependingspindle 102 of the upper section 78. The upper surface of the lowersection 80 includes a plurality of gas passage grooves. Morespecifically, a pattern of curvilinear distribution grooves 108 extendbetween a plurality of gas flow passages 110 and a central circulardelivery groove 112. Each of the grooves 108 and 112 is generallysemicircular in cross section and has a depth approximately equal tohalf the thickness of the lower section 80. Each of the gas flowpassages 110 opens downwardly into shallow spider arm cavities 114.

With reference to FIGS. 3 and 6, the spider assembly 40 is described inmore detail. The central hub 76 comprises a generally hollow cylindricalmember having a vertical through bore extending from a lower surface 116to an upper surface 118. The through bore comprises a lowershaft-receiving tapered portion 120, a central gas plenum 122, and anupper thermocouple channel 124. The lower tapered portion 120 receivesthe tapered upper end of the hollow drive shaft 36, the two elementshaving identical taper angles to fit snugly together. The thermocouplechannel 124 receives the central thermocouple 72 which extends upwardinto the thermocouple cavity 104 in the upper section 78 of thesegmented susceptor 42. The gas plenum 122 includes a plurality ofapertures 126 aligned with each of the support arms 74. In this respect,the support arms are hollow, with an interior defining sweep gaspassages 128. The upwardly directed terminal ends of the arms 74 arereinforced by annular lips 130. The lips 130 are sized to fit closelywithin the shallow arm-receiving cavities 114 in the underside of thelower section 80. The shaft 36 rotatably drives the spider assembly 40which, in turn, drives the susceptor 42 by the registration between thelips 130 and the shallow cavities 114 in the underside of the lowersection 80.

In an alternative embodiment, the curved arms of the spider assembly 40may be replaced by a pair of perpendicularly disposed tubes. That is,for each of the three arms, a first tube may extend radially outwardfrom the central hub 76 and couple with a second larger tubeperpendicular thereto and extending upward to fit closely within the armreceiving cavities 114. This arrangement can be visualized somewhat likea corncob pipe. The first tubes of each arm may radiate horizontallyfrom the hub 76 or may be slightly upwardly angled. Utilizing straightcylindrical sections, rather than a curved quartz tube, is lessexpensive to manufacture.

Referring back to FIG. 2, the spacers 100 may take several shapes. Inone preferred embodiment, seen in detail in FIG. 2a, the spacer 100 isin the form of a pin comprising an elongated upper portion 132 having asmall rounded head. A base 134 sized larger than the elongated portion132 fits within the counter-bored hole 98. The base 134 rests on theupper surface of the lower section 80. The heads of the elongatedportions 132 of the multiple spacers 100 terminate at the same height toprovide a planar support surface for the wafer 44. The upper portion ofthe counterbored holes 98 is approximately 0.062 inches in diameter andthe spacers 100 fit therein. The spacers 100 should preferably space awafer above the recess in a range of about 0.010 to about 0.200 inches;or more preferably in a range of about 0.060 to about 0.090 inches; andmost preferably the spacers 100 support the wafer 44 over the floor ofthe recess, a height of about 0.075 inches. This is about three timesthe thickness of a typical wafer. This spacing is significantly greaterthan the deviation from flatness of the susceptor or wafer which is inthe order of 0.005-0.010 inches. Also the spacing is much greater thanthe depth of a grid on the upper surface of a prior art susceptor whichhad been designed to optimize thermal contact between the susceptor andwafer while also facilitating wafer pickup. In a preferred embodiment,the depth of the recess 92 and spacer 100 height is such that the topsurface of the wafer 44 is in the plane of the outer ledge 84 tominimize any irregularity or transition and smooth gas flow thereover.Alternatively, the ledge 84 might be formed above or below the top ofthe wafer 44 as desired.

In an alternative embodiment, seen in FIG. 2b, the spacer 100 takes theform of a sphere 136 which fits within a cradle 138 formed in the uppersurface of the upper section 78. The spacer 100 may even be formedintegrally in the upper section 78. Desirably, the upper wafercontacting portion of the spacer 100 is rounded or terminates in a pointto minimize contact area with the wafer.

FIG. 2c, however, illustrates an alternative pin head configuration thatis useful with systems in which the wafer is dropped a short distancewhen being placed on the pins. That is, in one wafer transport system,the wafer is held by use of a so-called Bernoulli wand wherein a waferis held from above by radially outward gas flow, without the wafer uppersurface being touched by the wand. After a wafer is moved into positionslightly above a susceptor, the gas flow is interrupted and the waferfalls onto the spacers. While the fall distance is very slight, there issome possibility of a spacer pin with point contact chipping or marringthe surface of the wafer contacting the spacer. To minimize thatpossibility, the pin head of FIG. 2c has a flat upper surface 139 withrounded shoulders 139a. Preferably, the diameter of the flat area is inthe range of about 0.025" to 0.045", or the entire upper surface ofabout 0.055" could be flat. It is also desirable that the flat surface139 be polished to remove roughness on the surface that could possiblydamage the wafer.

The fixed spacers 100 define a planar support platform or stand for thewafer 44 to space the wafer above the segmented susceptor 42, and inthis respect at least three spacers are required, although more thanthree may be provided. Preferably, the spacers 100 are manufactured of aceramic or naturally occurring or synthetically fabricated sapphire,sapphire being a single crystal structure derived from aluminum oxide.In an alternative configuration, the spacers 100 may be formed ofamorphous quartz, although this material may eventually devitrify fromthe repeated thermal cycling within the reaction chamber 20. Furthermaterials which may be used for the spacers include monocrystalline orsingle crystal quartz, silicon carbide, silicon nitride, boron carbide,boron nitride, aluminum nitride, and zirconium carbide, or otherhightemperature resistant material capable of withstanding the extremetemperatures and the chemical environment in the wafer processingchamber. Any of these materials may additionally be coated with Si, Si₃N₄, SiO₂ or SiC to protect the spacers from deterioration from exposureto process gases.

To prevent back-side contamination of the wafer 44 from reactant gasesentering between the wafer and the susceptor 42, a novel sweep gassystem is provided. The system also preheats the gas which contacts thewafer and which if not heated would cause localized cooling and possibleareas of slip on the wafer. More particularly and with reference to FIG.2, the sweep gas enters the wafer support system through the hollowdrive shaft 36 and into the plenum 122, as indicated with arrow 140. Thegas is then distributed through the apertures 126 and into the sweep gaspassages 128 within the arms 74. The gas continues in an inlet flow 142into the gas flow passage 110 through the lower section 80. Thedistribution grooves 108 along with the lower surface of the uppersection define gas channels between the upper and lower sections 78, 80.Referring to FIG. 5, the gas flows along the channels following thevarious distribution grooves 108 to finally reach the circular deliverygroove 112, thereafter exiting through the sweep gas outlets 96, asindicated by arrow 144. The gas flow through the distribution grooves isshown by arrows 146. The gas flow into the delivery groove 112 is shownby arrows 148. The specific arrangement of the distribution grooves 108may be different than that shown in FIG. 5. The arrangement shown helpsreduce temperature nonuniformities through the lower section 80 andthrough the segmented susceptor 42 as a whole by channeling the sweepgas in a circuitous and symmetric path through the lower section.Desirably, the grooves 108 traverse a nonlinear path from the gas flowpassages 110 to the central circular delivery groove 112 and sweep gasoutlets 96.

The circular delivery groove 112 is formed directly underneath thecircular pattern of sweep gas outlets 96. As seen in FIG. 4, the evendistribution of gas through the groove 112 ensures that the sweep gasflow 148 leaving the outlets 96 is axisymmetric about the center of thesusceptor 42 in a radially outward direction. In this manner, anyreactant gas which might enter between the wafer and the susceptor isswept radially outward from underneath the wafer. Desirably, a flow rateof less than 5 standard liters/minute of sweep gas through the hollowshaft 36 and segmented susceptor is utilized, and a flow rate of lessthan 3 standard liters/minute is preferred.

Although other gases may be substituted, hydrogen is preferred as it iscompatible with many CVD processing regimes. As a result of theexcellent control over the backside of the wafer through the use of thepurge gas, wafers with double-sided polishing can be processedsuccessfully, unlike a system with the wafer in contact with thesusceptor.

The present invention includes the mass flow controller 39 to regulatethe flow of sweep gas through the hollow shaft 36 and segmentedsusceptor for different processing pressures. That is, some processesare at atmospheric pressure, and some are at reduced pressure. In thecase of a fixed restriction to control flow, a reduced pressure processwill tend to increase the flow of gas through the sweep gas outlets 96as compared to an atmospheric process, all other variables remaining thesame. Thus, the mass flow controller 39 operates independently from theprocess pressure to ensure a constant flow of less than 5 standardliters/minute.

FIGS. 7 and 8 illustrate another wafer support system 22' which utilizessome of the same elements as the wafer support system 22 shown in FIG.2. More particularly, the spider assembly 40 and lower section 80 of thesegmented susceptor 42' are identical to those shown and described withreference to the first embodiment. The segmented susceptor 42', however,includes a modified upper section 78', with an outer ring 82' comprisingan upper ledge 84' and a lower skirt 86'. The upper ledge 84' is sizedsimilar to the ledge 84 described with respect to the first embodimentand terminates in a circular step 88' leading to a circular recess 92'.The circular recess 92' extends radially outwardly past the lowersection 80. In relative terms, the lower skirt 86' is substantiallygreater in the radial dimension in comparison to the skirt 86 describedfor the first embodiment, yet the step 90' is sized the same as the step90 in the first embodiment. This allows the upper section 78 to receivethe annular lower section 80 therein, just as in the first embodiment.

In a departure from the first embodiment, as seen in FIG. 7, thesusceptor 42' includes a plurality of spacers in the form of supportpins 150 circumferentially distributed about a circle around the centralaxis of the susceptor 42' in the region between the upper step 88' andthe lower step 90'. More particularly, the pins 150 extend withinstepped cavities 152, extending through the upper section 78' from therecess 92' to the extended skirt 86'. The pins 150 shown are somewhatdifferent than the first two embodiments described with respect to FIGS.2a and 2b, and comprise simple cylindrical elements having rounded headsin contact with the wafer 44'.

An alternative embodiment of gas passage grooves through the susceptoris shown in FIGS. 9 and 10. As before, the spider assembly 40 supports amodified susceptor having an upper section 162 and a lower section 164.The lower section 164 includes three gas passages 166 opening downwardlyto receive the upper ends of the spider assembly arms 74. In thisrespect, the locations of the sweep gas inputs are in the same locationas with the first two susceptor embodiments 42 and 42'. From there,however, distribution grooves 168 in the upper surface of the lowersection 164 extend radially outward to an outer circular groove 170.Secondary grooves 172 channel the sweep gas radially inward to intersecta series of concentric circular delivery grooves 174a, 174b and 174clocated at spaced radii. Each secondary groove 172 preferably lies alonga line which bisects the included angle defined between each pair ofdistribution grooves 168.

Looking at FIGS. 9 and 10, the upper section 162 includes a plurality ofgas outlets arranged in a series of concentric circles corresponding tothe circular delivery grooves 174a, 174b and 174c. More particularly, afirst group of outlets 176a lie along an inner circle 178a at the sameradius of the smallest delivery groove 174a.

Likewise, two more groups of outlets 176b and 176c are arranged aboutouter concentric circles 178b and 178c, respectively, which correspondto the outer delivery grooves 174b and 174c.

Four outlets 176 are shown distributed evenly about each of the circles178a,b,c, but more or less may be provided. Furthermore, thecircumferential orientation of the outlets 176 may be staggered betweenthe circles 178 as shown. With four outlets 176 per circle 178, eachpattern of outlets is rotated 30° with respect to one of the otherpatterns. Alternatively, for example, eight outlets 176 per circle 178evenly distributed and staggered would mean that each pattern of outletsis rotated 15° with respect to one of the other patterns. The staggeringbetween patterns creates a more effective gas sweep under the wafer, asshown by arrows 180, than if the outlets 176 were aligned.

In another variation, the upper section 162 may be used with the lowersection 80 described above with respect to FIGS. 3 and 5 as long as theinner circle 178a of outlets 176a aligns with the circular deliverygroove 112. In that case, the outer circles 178b,c of outlets 176b,cwould not be used. Additionally, the lower section 164 may be used witheither of the above described upper sections 78, 78' as long as theinner delivery groove 174a aligns with the circular pattern of outlets96, 96'. In that case, the outer delivery grooves 174b,c would not beused. Of course, other variations are contemplated.

The separation between the wafer 44 and the segmented susceptor 42, aswell as the minimal direct support provided by the three spacers 100,effectively decouples the wafer and susceptor from heat conductiontherebetween. The wafer 44 temperature is thus influenced primarily fromradiant heat flux provided by the lamps surrounding the chamber.

The spider assembly 40 is preferably constructed of quartz to provide atransparent support to the underside of the susceptor 42 to minimize theobstruction of radiant heat emitted from the lower heat lamps. Althoughquartz is preferred, other materials having a relatively highcoefficient of radiant heat transmission may be utilized. To constructthe spider assembly 40, the hub 76 is first machined into the propershape. The tubular arms 74 are bent from straight portions and attachedto the hub 76 by welding, for example. Heat treating and fire polishingreduce internal stresses in the quartz.

FIG. 11 illustrates a top plan view of another wafer support system 200of the present invention again comprising a segmented susceptor 202having a concentric recess 204 in a top surface, and a plurality ofwafer support spacers 206 positioned within the recess.

With reference FIG. 12 which illustrates a top section 208 of thesegmented susceptor 202, the shallow recess 204 is defined around itsouter perimeter by a circular step 210 leading to a ledge 212 whichforms the uppermost surface of the susceptor. The construction is, inmany respects, similar to the susceptors previously described.

In a departure from the previously described susceptors, the segmentedsusceptor 208 includes two concentric circles of sweep gas outlets. Anouter circle of twelve sweep gas outlets 214 surrounds an inner circleof twelve sweep gas outlets 216. It can be readily seen from FIG. 12that the outer sweep gas outlets are distributed about the center of thesegmented susceptor 208 at intervals of 30°, or at 1:00, 2:00, etc. Theinner circle of sweep gas outlets 216, on the other hand, are offset 15°rotationally with respect to the outer circle, and thus occupyrotational positions at 12:30, 1:30, etc., intermediate the outer circleof outlets. This increased number of sweep gas outlets and staggeredrelationship of the concentric circles increases the uniformity of sweepgas underneath the wafer and improves performance therefor; as waspreviously described with respect to FIG. 9.

FIG. 11 illustrates in dashed line, an interface 219 between the topsection 208 and a bottom section 218 of the segmented susceptor 202, thebottom section being seen in top plan view in FIG. 13. The outerperiphery of the bottom section 218 is substantially circular, exceptfor three flats 220 disposed at 120° intervals therearound. The outerperiphery of the bottom section 218 fits within a similarly shaped lowerstep 222 of the top section 208, as seen in dashed line in FIG. 12, andin cross-section in FIG. 14. The flats 220 of the bottom section 218cooperate with inwardly-facing flats 224 formed in the lower step 222 torotationally orient the top section 208 with the bottom section 218. Thebottom section 218 further includes a small central through bore 226within which a downwardly depending hub or spindle 228 of the topsection fits.

The underside of the bottom section 218 includes three shallow spiderarm cavities 230, similar to those previously described. The cavities230 communicate with vertical gas flow passages 232 leading to aplurality of gas distribution grooves 234 formed in the upper surface ofthe bottom susceptor section 218. As seen in FIG. 13, each gas flowpassage 232 communicates with diverging grooves 234 which travelcircuitous paths extending first radially outwardly, thencircumferentially adjacent the periphery of the susceptor lower section,and finally generally radially inwardly toward the center of the bottomsection 218. In this manner, sweep gas flows substantially through theentire susceptor in a generally axisymmetrical pattern to provide evenheat transfer to the sweep gas from the hot susceptor, and visa versa.

Both gas distribution grooves 234 intersect a continuous outer circulardelivery groove 236 concentrically formed in the bottom section 218.From the outer groove 236, a plurality of angled spokes 238 lead to aninner circular delivery groove 240, again concentrically formed in thebottom section 218. Although the gas distribution grooves 234 are showncontinuing directly into each of the spokes 238, other arrangements arepossible. Furthermore, the spokes 238 are shown intersecting the innercircular delivery groove 240 at generally tangential angles, but mayalso be connected at other more direct radial angles. The gas flowpassages 232 are located radially outward from the sweep gas outlets 216and the gas distribution grooves 234 desirably traverse a nonlinear paththerebetween, preferably longer than a direct line between any of thepassages 232 and outlets 216, and most preferably in a circuitouspattern such as the one shown.

The inner circular delivery groove 240 lies directly underneath theinner circle of sweep gas outlets 216 when the top section 208 iscoupled over the bottom section 218. Likewise, the outer circulardelivery groove 236 lies directly underneath the outer circle of sweepgas outlets 214. This arrangement allows for an even pressure and supplyof sweep gas to all of the outlets 214, 216 in the top surface of thesegmented susceptor 208. The pressure created between the top and bottomsections 208, 218, is reduced somewhat from previously describedembodiments by the increase in the number of sweep gas outlets 214, 216,and by the reduction in size of the inlet gas flow passages 232. Morespecifically, the inlet gas flow passages 232 have a diameter ofapproximately 0.060 to 0.070 inches. FIG. 11 illustrates the gas flowfrom the passages 232 through the distribution grooves 234 with arrows242.

In a departure from previous embodiments, and as seen in FIG. 12, eachof the spacers 206 is supplied with purge gas from one of the gasdistribution grooves 234 via a purge channel 244. These purge channelsare seen in cross-section in FIG. 14 and extend from the respective gasdistribution groove 234 directly to the spacer 206. In this manner, acontinuous supply of purge flow, indicated at 246, is supplied to theregions surrounding each spacer 206. Each of the spacers 206 fits withinan aperture 250 formed in the top surface of the recess 204. A clearanceis provided between the spacer 206 and its aperture 250 so that thepurge gas may flow upward therearound and protect the spacer fromdeposition gases. More particularly, when the wafer 248 is not present,the sweep gas through the outlets 214, 216 flows generally upward intothe reaction chamber, rather than outward around each of the spacers.This leaves the spacers 206 unprotected from etch or capping gases. Thespacer is defined by a lower cylindrical base 252 and upper elongatedcylindrical pin 254 having a rounded upper surface. The pin portion 254is undersized with respect to the aperture 250 to allow the purge flow246 therethrough. In one embodiment, the pin 254 has a diameter ofbetween 0.050 and 0.055 inches, while the aperture 250 has a diameter ofbetween 0.062 and 0.067 inches.

The present invention provides a susceptor combination enablingselection of different upper sections depending on the wafer size to beprocessed. Such a combination is especially useful in the reactionchamber 20 having the support plate 32. As mentioned above, thesusceptor preferably has a constant outer diameter to fit within thering 62, and aperture 33 in the support plate 32. As the upper sectiondefines the outer perimeter of the susceptor, it will by necessity havea constant diameter while the wafer recess varies in size to accommodatethe different wafer sizes. The bottom shape of each of the uppersections is designed to mate with a single lower section, which reducescosts somewhat. FIGS. 11-17 illustrate four different susceptorcombinations 200, 258, 278 and 300 for four different wafer sizes. Othersizes of wafers may of course be accommodated by such a combination, themaximum size only being limited by the outer diameter of the susceptor.

FIG. 15 illustrates a second version of a top section 260 of the wafersupport system 200. The bottom section is the same as was described withrespect to FIGS. 11-14. Indeed, an interface 262 between the top section260 and the bottom section 218 is the same as previously described, andthe gas distribution grooves 234 in the bottom section are in the samelocation. The top section 250 differs from the earlier described versionby a reduced diameter recess 264. The recess 264 is defined by thecircular step 266, which in turn creates a larger radial dimension forthe ledge 268. The top section 260 is adapted to support smaller sizedwafers within the recess 264. In this respect, a plurality of spacers270 are positioned at 120° intervals around the center of the susceptorand at radial distances which provide adequate support for wafers ofapproximately 150 millimeters. To connect the purge gas grooves 234 withthe spacers 270, shortened purge channels 272 are provided.

FIG. 16 illustrates a third version of a top section 280 of the wafersupport system 200. Again, the bottom section is the same as before withthe interface 282 between the top and bottom sections being the same.The top section 280 includes an enlarged ledge 284 terminating in acircular step 286. The recess 288 thus formed is sized to receive wafersof approximately 125 millimeters in diameter. Purge channels 288 lead toapertures surrounding the captured spacers 290 at radial dimensionssufficient to support the reduced-size wafers. It will be noted that thegas distribution grooves 234 extend radially outward from the recess266, and then continue inward to the circular delivery grooves.

In a fourth version of the top section 302, seen in FIG. 17, the step304 is even further moved inward, reducing the recess 306 to a sizesufficient to support 100 millimeter wafers. Again, the interface 308remains in the same location, as the bottom section of susceptor 300 isidentical to that previously described. The outer ledge 310 is greatlyenlarged in this embodiment. Three spacers 312 are provided at 120°intervals around the center of the susceptor, and three associated purgechannels 314 connect the gas distribution grooves 234 thereto. It willbe noted that the radial positions of the spacers 312 are within thecircle created by the three gas inlet apertures in the bottom surface ofthe susceptor. Indeed, the gas distribution grooves 234 extend radiallyoutward from the recess 306, and then continue inward to the circulardelivery grooves. Furthermore, the location of the support arm-receivingcavities is just outside of the recess 306, and is thus outside of thewafer when positioned on the susceptor 300. The ledge 310 surroundingthe recess 306 extends outward radially from the wafer for at least halfthe wafer diameter.

Referring now to FIGS. 21A-21E, there is illustrated another variationof a segmented susceptor. FIG. 21A illustrates a top section 408 havinga shallow recess 404 defined around its outer perimeter by a circularstep 410 leading to a ledge 412 which forms the uppermost surface of thesusceptor. A circle of spaced sweep gas outlets 416 are located fairlyclose to the circular step 410. In the arrangement shown 24 outlets areprovided. Positioned closer yet to the step is a circle ofcircumferentially-spaced support pin or spacer holes 450. With thisarrangement, the wafer support pins or spacers will engage theundersurface of a wafer adjacent its outer periphery. Since a wafertypically has an alignment flat or notch on its outer periphery, sixsupport pins are provided instead of three as in the earlierarrangements. Thus, even if a wafer alignment flat or notch is alignedwith a support pin so that little or no support is provided by thatparticular pin, the wafer is still adequately supported by the otherfive.

As seen in FIG. 21B, a susceptor lower segment 418 includes shallowspider arm cavities 430, similar to those previously described. Thecavities communicate with vertical gas flow passages 432 leading to aplurality of gas distribution grooves 434 formed in the upper surface ofthe bottom susceptor section 418. As seen in FIG. 21B, each gas flowpassage 432 communicates with groove sections which travel circuitouspaths leading to an outer annular groove 435 at circumferentially spacedlocations. One segment of each path first extends radially outwardly andthen turns inwardly to form somewhat of a horseshoe shape, and thenextends circumferentially and radially outwardly to form a secondhorseshoe-shaped portion before intercepting the outer groove 435. Theother section of the path first extends radially inwardly and thencurves radially outwardly and then circumferentially before intersectingthe outer groove 435.

As seen from the fragmentary portion of the upper segment 408 in FIG.21B and as further illustrated in FIGS. 21C, D, and E, the outer groove435 is located beneath the circle of sweep gas outlets 416. Bypositioning the sweep holes so close to the outer periphery, the risk ofbackside deposition is greatly reduced. Further, the gas passages 434together with the increased number of gas outlets 216 increases thesweep gas flow volume. Also, reducing the spacing between the peripheryof the wafer and surrounding recess wall to about 0.10 inch furtherminimizes the possibility of deposition gas entering beneath the wafer.

By positioning the support pin cavities 430 adjacent the outer peripheryof the recess in the susceptor, the upper surface of the wafer supportpins 446 engage the lower surface of the outer periphery of a wafer 448in an outer area referred to as an exclusion zone 449. This zone doesnormally not become a part of a semi-conductor circuit chip. Hence, anyslight marking to the undersurface of a wafer that might be caused by asupport pin is inconsequential.

FIG. 21D illustrates a situation in which the wafer is formed with analignment notch 451. Even with that arrangement a pin will engage thewafer if the notch should happen to be aligned with wafer, so long asthe wafer is centered and the gap between the wafer edge and thesurrounding recess is small. In fact, with a small enough gap, the pinwill engage the wafer even if not centered on the susceptor.

FIG. 21E illustrates the situation in which a wafer flat 453 is alignedwith a support pin 446. As can be seen, the spacer pin is not actuallyengaging the wafer, but this is of no consequence since the wafer issupported by five other spacers.

FIGS. 22A and 22B, show another segmented susceptor assembly wherein alower susceptor section 518 is shown with a circular groove 537 whichintersects three spider arm flow passages 532. A shallow annular recess539 extends from the groove 537 out to a circular edge 541 close to theperiphery 518A of the lower section. More specifically, the edge 541 islocated just radially beyond a circle of circumferentially-spaced sweepgas outlets 516 adjacent the periphery of an upper susceptor segment512, a fragment of which is shown in FIG. 22A. Six support pins 546 arealso conveniently shown in FIG. 22A. The circle of outlets 516 shown inthe arrangement of FIGS. 22A and B is essentially the same as that shownin FIG. 21A except that three times as many outlets are illustrated.Thus, in this arrangement, 72 outlets are utilized for a susceptoradapted to receive a 200 mm wafer 548. The exact number of the outletscan of course be varied, but it is advantageous having so many sweep gasoutlets and having the shallow but large area annular recess 539 forfeeding sweep gas to those outlets, as shown by the arrows in FIG. 22A.The increased gas flow greatly reduces the risk of deposition gasreaching the backside of the wafer.

FIGS. 23A, B, and C, illustrate arrangement which can be similar to anyof the arrangements previously described except that it employs a spacerin the form of a ring rather than a plurality of pins. Morespecifically, there is illustrated a thin generally flat spacer ring 615positioned in a shallow recess 604 in the top section 608 of a segmentedsusceptor 602. The outer perimeter of the ring 615 is located justwithin the edge of the recess as defined by a circular step 610 leadingto a ledge 612, which forms the upper surface of the susceptor. The ring615 extends inwardly to about the location occupied by the support pinsin the arrangement of FIGS. 21 and 22. As seen from FIG. 23B, the uppersurface 615a of the ring 615 is not quite horizontal. Instead, it slopesor angles downwardly in a radially inward direction. Thus, the radiallyouter portion is the thickest vertically. The vertical thickness of thering in the area engaged by the wafer is equal to the height of theportion of support pins that protrude above the recess in the uppersection of the susceptor in the above-described arrangements. The waferis effectively thermally decoupled from the susceptor and suitablypositioned with respect to the upper surface of the outer ledge of thesusceptor. With only the outer perimeter of the lower surface of thewafer 648 engaging the ring 615, the ring avoids or minimizes anymarkings on the backside of a wafer. Moreover, any insignificant effectwould be within the wafer exclusion zone and be confined to the edgeprofile of the wafer.

A plurality of radially extending passages or grooves 617b are formed inthe upper surface of the spacer ring 615. Thirty-two passages areillustrated in a susceptor for receiving 200 mm wafers. As seen fromFIG. 23A, these passages are circumferentially-spaced and provideoutlets for the sweep gas, as shown by the arrows in FIG. 23B. The ringbody between and around those passages blocks deposition gas flow intothe backside of the wafer.

FIG. 23C illustrates a semi-circular cross-sectional shape of thepassage 615b, but of course other configurations may be employed. Thecross-sectional area and the number of passages are selected to providethe desired flow, consistent with gas provided through passages 632 in alower susceptor section 618, seen in FIG. 23B. The passages 632 areshown for convenience in FIG. 23A even though no other details of alower susceptor section are shown. As mentioned above, any of the sweepgas arrangements described above may be employed with the ring conceptof FIGS. 23A-C. In fact, the support ring can be employed with uppersusceptor segments designed to receive support pins inasmuch as the pinholes do not interfere with the use of the ring and do not have asignificant effect on the sweep gas system. Thus, a user can employeither approach.

The ring can be conveniently made of the same material as the supportpins or the susceptor.

In testing the wafer support system described above, it has been learnedthat certain aspects of the reactor system are particularly important inobtaining satisfactory results. FIG. 18 illustrates a rectangularchamber having a flat upper wall 324 and a flat lower wall 325 in aninlet section and a flat lower wall 326 which is stepped down from thewall 325 by a flat vertical wall 327. The horizontal walls 324, 325 and326 are joined by flat vertical side walls 328 and 330 to create achamber having a shallow rectangular inlet section and a deeperrectangular section adjacent to it in which is positioned a susceptor382 and a temperature compensation ring 362.

It is preferred that the ring 362 surrounding the susceptor have agenerally rectangular exterior shape as shown in FIG. 19. Further, it isalso desirable that the radiant heating lamp banks 351 and 352 above andbelow the upper and lower walls 324 and 326 of the quartz chamber inFIG. 18 define an exterior shape that is generally rectangular andconform to that of the ring, so that the projected radiant heat patternor column is likewise generally aligned with the ring. That is, the heatis primarily directed to the ring and the susceptor area rather thanbeing directed to the quartz walls adjacent the ring. This heatingarrangement is highly efficient and promotes uniform temperature anddeposition across the ring and the susceptor. Incidentally, the spotlamps 353 beneath the central portion of the susceptor are considered tobe part of the lower lamp bank 352.

The ring is supported on a suitable quartz stand 356 resting on thebottom of the chamber. Alternate supporting arrangements may be employedsuch as utilizing ledges or fingers extending from the adjacent quartzstructure. This configuration of the ring and the radiant lamps has beenfound to work particularly well in a chamber having a generallyrectangular cross section formed by the flat upper and lower walls 324and 326 and vertical side walls 328 and 330.

The combination of the rectangular chamber and the rectangular ringsimplifies the process gas flow across the wafer. With the rectangularring, the process gas introduced through an injector such as at 46 inFIG. 1, is depleted generally uniformly across the width of the chambersuch that the velocity profile of the process gas may be generallyuniform across the chamber as schematically indicated by the arrows 331in FIG. 19. Consequently, a minimum of carrier gas is required with therectangular ring and the rectangular chamber cross section, since thereis no need to increase flow in the center. The reduced carrier gas flowmeans less cooling effect on the wafer. This is important for a waferspaced from the susceptor, since the spaced wafer is more responsive tothe cool gas flow than is a wafer supported directly on the susceptor.The volume of hydrogen gas has been reduced by about 75% in a prototypesystem. Stated differently, the ratio of the carrier gas to thedeposition gas has been reduced from a minimum of about 15 to 1 to aminimum of about 5 to 1.

With the wafer substantially thermally decoupled from the susceptor, ithas been found to be quite sensitive or responsive to nonuniformity inthe heat output of the lamp banks. For example, the spacing between thelamps and the distance of the lamp banks from the wafer and thesusceptor 382 affect the uniformity of the heat pattern obtained on thewafer. Thus, with the wafer spaced from the susceptor 382, it has beenfound desirable to increase the distance between the wafer and the upperlamp bank 351 from that employed with a wafer positioned directly on thesusceptor. Likewise, it has been found desirable to increase thedistance from the susceptor to the lower lamp bank 352. But it has beenfound desirable to increase the space between the wafer and the upperlamp bank 351 more than the space between the lower lamp bank 352 andthe susceptor.

Common to all the various arrangements disclosed, the wafer is supportedin a reactor largely thermally decoupled from the susceptor. That is,the wafer is supported on spacers or pins that space the wafer asubstantial distance above the susceptor. The pins have minimal contactwith the wafer. The sweep gas is preheated by way of the novel susceptordesign so that it has an insignificant effect on the temperature of thewafer but yet effectively prevents process gases from depositing on thebackside of the wafer. Since the wafer is essentially decoupled from thesusceptor, the wafer can heat more quickly as compared to a systemwherein the wafer is in contact with the susceptor.

The lamp banks 351 and 352 are controlled by a suitable electroniccontroller schematically shown at 390 in FIG. 18. The controllerincludes a transmitter component that receives signals from thetemperature sensors in the ring surrounding the susceptor and from thesensor located at the center of the lower side of the susceptor. Thesetemperature signals are transmitted to heater control circuitry.Additionally, temperature control information such as varioustemperatures settings desired for a particular deposition cycle isinputted to the heater control circuitry. That information is thenprocessed by the control circuitry, which generates control signals thatcontrol power to the heating assemblies. Further details of such asystem, are disclosed in U.S. Pat. No. 4,836,138, which is expresslyincorporated herein by reference.

In that earlier system, some lamps from the upper and lower lamp banksare controlled together as a zone that would be adjusted as a unit. Thatis, the power ratio was fixed so that if the power was increased to alamp in the upper bank, a corresponding power increase was provided to alamp of that particular zone in the lower bank as well. The ratio isadvantageously fixed by applying the temperature control signal for agiven lamp bank through a pre set ratio potentiometer that modifies thecontrol signal before it is applied to the lamp bank. The other lampbanks advantageously have their control signals modified using similarratio control circuitry, there by providing a pre set power ratiobetween the lamp banks within a zone. In this way the various zones canbe adjusted independently. One change to the system described in U.S.Pat. No. 4,838,138 has been made as a result of the wafer on spacersdesign. An analog ratio control has been added to the circuitry topermit the lamp bank power ratio between the upper and lower lamps of aparticular heating zone to be adjusted at various points during theprocess as a result of the thermal decoupling of the wafer from thesusceptor. This is advantageously accomplished in the current system byadding a dynamically controllable ratio potentiometer in series with thepre set ratio potentiometer for the lamps in the upper lamp bank withina zone. Thus, the control signal for the upper lamp bank within the zonemay be changed using the dynamically controllable potentiometer. Becausethe total power applied to the lamps in that zone remains about thesame, when the power to the upper bank lamps within the zone is changed,the power to the corresponding lamps 352 in the lower bank is changed inthe opposite direction. Thus the power ratio between the two is changed.This enables the temperature of the susceptor and the wafer to bemaintained close together even though they are physically spaced.

Referring more specifically to the heating system disclosed in U.S. Pat.No. 4,838,138, lamps 48B and 48C of FIG. 6 form a central heatingportion of an upper lamp bank, and the lamps 78B and 78C form a centralportion of a lower bank. The ratio of power between the upper and lowerbanks was changed utilizing the analog ratio control by changing thepower applied to the upper bank lamps 48B and 48C, while the total powerapplied to the lamps 78B, 78C, 48B and 48C remains about the same. Thisresults in a change to the power to the lower bank in the oppositedirection.

An example of utilizing the analog ratio control is illustrated in thegraph of FIG. 20. The solid line illustrates a time temperature recipefor the processing of a semi-conductor wafer. The solid line indicates awafer being loaded into a reactor with the lamps set to provide astarting temperature of 900° C. The temperature is maintained at thatlevel for about 30 seconds. Additional heat is then applied ramping thetemperature up to about 1150° C. in about 70 seconds. The wafer is thensubjected to a bake or etch step at that level for about a minute. Thetemperature is then allowed to decrease to a deposition temperature ofabout 1050° C. with the cooling occurring in about 30 seconds. Thetemperature is maintained at 1050° C. for about 70 seconds in apredeposition phase followed by about 70 seconds while the deposition isoccurring on the wafer. The wafer is then allowed to cool to about 900°C. for a similar time. The cycle is then complete and the wafer isremoved at the 900° C. level.

As explained above, the ratio of the heat between an upper bank of lampsand a lower bank of lamps has been kept at a predetermined ratio whenthe wafer being processed is supported directly on the susceptor. Thatmethod is satisfactory with the wafer positioned on the susceptorinasmuch as the temperature between the susceptor and the wafer arelargely the same throughout the cycle. However, with the wafer spacedabove the susceptor, it is desirable to change the ratios between theupper and lower heating banks in the central portion of the wafer duringthe cycle. The broken line of FIG. 20 provides an example of the analogratio control. The ratio percentage change is illustrated on theright-hand scale of the chart of FIG. 20. At the start of the cycle, theratio is shown at a zero percentage variation, meaning that the lampsare at what might be termed a steady state condition or the fixed ratioposition. This does not mean that the power between the upper and lowerbanks is necessarily the same. As an example of an operating system theupper lamps received about 48% of the power and the lower lamps about52%. With a wafer supported in direct contact with a susceptor, thepower ratio would simply remain on the zero or steady state line.However, that is not satisfactory with the wafer spaced from thesusceptor.

It is desirable to maintain the temperature between the wafer and thesusceptor approximately the same during the heating cycle. Since thewafer is spaced above the susceptor and has less mass than thesusceptor, it heats more quickly then the susceptor. Thus, thepercentage of heat required by the wafer is reduced during the phase ofthe cycle in which the temperature is ramped up from 900° C. to 1150° C.Thus, the broken line of the graph shows that the percentage of powerapplied to the upper lamps is decreased to a ratio about 20% below thesteady state or zero change condition. As mentioned above, the totalpower applied to the lamps is about the same as it would be if the ratiowere not changed, and hence, this results in an increase in thepercentage of the power being applied to the lower lamps. With thischanged ratio, the temperature of the wafer and the susceptor remainsubstantially the same as the temperature is ramped up to the 1150° C.level. While the temperature is maintained at that level for the bake oretch phase, the variable ratio control is returned to the zero or steadystate ratio as shown on the graph.

When it is then desirable to cool the wafer from 1150 to 1050° C., thepower is reduced; but some power is continued to control the cooling.Since the wafer spaced from the susceptor cools more quickly than thesusceptor, the ratio between the upper and lower lamps is changed byreducing the power to the upper lamps a lesser percentage than to thelower lamp to maintain the wafer at the susceptor temperature. As shownon the broken line, the percentage of power to the upper lamp isincreased so that the ratio is increased by about 20% to the upper lamp.While the wafer is maintained at that 1050° C. level, the power ratio isreturned to the steady state condition so that at the time that thepredeposition phase is over and the deposition phase is to commence, thepower ratio is at the so-called steady state condition. Afterdeposition, it is desirable to allow the wafer to cool to the 800° C.level; and hence again, the ratio is changed by increasing thepercentage of the power to the upper lamps by about 20%. When the 800°C. level is reached, the power percentage is decreased with respect tothe upper lamp, allowing the ratio to return to its steady statecondition. It should be kept in mind that the total power applied isapproximately the same and it is only the power ratio between the upperand lower banks which is being altered. The actual percentage changes,of course, have to be determined for the particular wafers beingprocessed and the particular temperatures and processes involved. Theanalog ratio control feature employs multiplier circuits to modify thepower signal to the upper lamps by the appropriate fraction to obtainthe desired result.

FIGS. 24 and 25 illustrate an arrangement similar to that in FIGS. 23A,B and C, but it includes a spacer ring 715 having a configurationdifferent from the spacer ring 615. Instead of being a flat ring with aplurality of circumferentially spaced grooves 615b in its upper surface,the ring 715 includes a central main body portion 715b having agenerally flat rectangular cross-section, as best seen in FIG. 25B. Aplurality of lands, lips or projections 715a extend upwardly from themain body portion 715b to form spacers for the substrate. In thearrangement shown in FIG. 24, six such lands are provided,circumferentially spaced at an equal angle α of approximately 60°. Asseen from FIG. 25A, the lands extend the complete radial thickness ofthe ring, but the upper surface of the land 25b is slightly sloped froma radially outer edge to a lower radially inner edge of the ring. Thisarrangement minimizes the contact between the substrate 648 to a veryslight line contact at the six land locations. Further, as can be seenfrom FIG. 25, the circumferential width of the land is very small,preferably only about 0.030 of an inch. The slope of the upper surfaceof the land is only about 2° from horizontal. Projections orprotuberances with other configurations can be employed instead of thelands.

The ring 715 is further provided with a plurality of feet 715c dependingfrom the main body portion 715b at circumferentially spaced intervals.More specifically, it can be seen from FIG. 24 that a pair of such legsstraddle a land 715a and are spaced from the land a circumferentialangle β of about 10°. This creates a total of 12 feet, two adjacent eachside of each land 715a. As seen from FIGS. 25A, 25B and 25C, the feetextend the full width of the main ring body 715a, except that the outerlower corners of the feet 715c are chamfered.

Spacers supporting a wafer above a susceptor have less resistance tothermal transport than the gas between the wafer and the susceptor.Thus, undesirable thermal gradients can be created within the wafer nearthe contact area. This is most significant with larger thermal gradientsthat may occur during rapid heat ramp-up of the system. An advantage ofhaving a land 715a circumferentially spaced from a foot 715c is that thethermal path between the susceptor and the wafer is much longer thanthat with a spacer extending directly between the two components. Or,stated differently, the thermal path from the bottom of one foot to thetop of an adjacent land is much greater than the height of the ringincluding the foot and the land. This in turn permits rapid heating of asystem, which of course improves productivity.

With the arrangement illustrated in FIGS. 24 and 25, the remainder ofthe susceptor can be formed utilizing any of the susceptorconfigurations of FIGS. 2-17, with or without spacer pins. That is, ifthe spacer ring is provided with a height equal to that of the spacerpins, the spacer pins need not be employed. Alternatively, the totalheight of the blocker or spacer ring can be slightly less than that ofthe spacer pins such that the substrate is supported by the spacer pins.

Various dimensions of the spacer ring 715 may be employed. For example,the height A of the land 715a in one prototype version for an 8 inchwafer is approximately 0.022 of an inch, with the central body portionbeing about 0.035 of an inch, and the feet being about 0.020 of an inch,for a total of about 0.077 of an inch. The thickness B of the centralbody portion 715b can be increased to decrease the area of the passagesbetween the lands and between the feet. In another configuration, themain body portion 715b is about 0.045, with the projection 715a beingabout 0.017 and the foot being about 0.015. It should be noted thatsince the diameter of the substrate is slightly less than the outerdiameter of the ring, the height of the land 715a at the area contactedby the periphery of the substrate is about the same as the height of thefoot. In another configuration, the central body portion 715b is about0.055 of an inch, with the upper and lower portions being about 0.010 ofan inch each. In yet a fourth configuration, the central body portionwas about 0.065 of an inch, with the upper and lower projections beingonly about 0.005. Thus, it can be seen by varying the dimensions of thering, the cross-sectional area of the passages between the ring and thesubstrate, and the ring and the susceptor, are correspondingly varied.

FIGS. 26 and 27 illustrate another configuration of a spacer or blockerring 815. As seen from FIGS. 26, 27, 27A and 27B, the ring includes amain body portion 815b having a generally rectangular cross-section, andincludes an upwardly extending continuous annular rib 815a which ispositioned approximately midway between inner and outer diameters of thering. The ring is further provided with a plurality of circumferentiallyspaced feet 815c that depend from the main body portion 815b. These feetare approximately the same as the feet 715c illustrated in FIG. 25. Thatis, in the arrangement illustrated, a pair of feet 815c are spaced fromeach other at an approximate angle θ of about 20°. Further, there aresix pairs of such feet circumferentially spaced approximately 60°,thereby creating a total of 12 feet.

The ring 815 is preferably used as a blocker ring in which the overallheight of the ring is less than that of the support pins or spacersdiscussed above, so that the substrate is supported on the spacer pinsrather than the blocker ring. In that sense, the ring 815 serves only toblock the inward flow of deposition gas and to further improve theaction of the sweep or purge gas by providing a thin annular passage orslit of only about 0.010 of an inch between the upper edge of the rib815a. Also provided are circumferentially spaced, vertically shortpassages between the feet 815c. In a preferred arrangement, the heightA¹ of the rib 815a is about 0.025 of an inch, the main body height B¹ isabout 0.030 of an inch, and the height C¹ of the foot is about 0.010 ofan inch for a total of about 0.065. When used with spacer pins thatcreate a gap of 0.075 inch, this created the 0.010 inch passage betweenthe rib and the substrate.

The radial dimension or width of the annular rib 815a is preferablyabout 0.025 inch; and as seen in FIG. 27B, it has a generally flatcentral portion with rounded shoulders.

To further block the gap between the substrate and the susceptor, theblocker ring feet 815c may be eliminated, creating a cross-sectionillustrated in FIG. 27C, wherein the main body portion is about 0.040inch.

FIG. 28 shows a blocker ring 915 having a cross section similar to thering 815 accept that an annular rib 915a is located adjacent the innerdiameter of the ring, thus giving the ring cross section somewhat of anL shape, with the radial dimension of the ring representing the long legof the L shape and the upwardly extending rib representing the shorterleg.

An advantage of the arrangement illustrated in FIGS. 26, 27, and 28 isthat with the ribs 815a and 915a spaced from the substrate, the ring issubstantially thermally decoupled from the susceptor so that there areno significant temperature discontinuities in the area of the susceptorabove the ring that might create slip. At the same time, since asubstantial portion of the gap is blocked by the ring, deposition gas isblocked from entering the area beneath the substrate. Related to that isthe fact that the velocity of the sweep gas is increased as it passes bythe ring, which further inhibits the flow of deposition gas beneath thesubstrate. The input gas flow into the gap from the passages through thesupport spider can be controlled to create the desired flow and apressure is maintained in the gap between the susceptor and thesubstrate that is greater than the pressure above the substrate. Thispressure differential, of course, maintains the flow of purge gas orsweep gas and prevents the flow of deposition gas on the backside of thesubstrate. The use of a blocker ring, such as in FIGS. 23-28, providesgood backside protection for the wafer with less gas flow than withoutthe ring. Gas flow at various low flow rates has provided good results.

While some of the spacer rings are described as having feet or legsprojecting downwardly from a main body portion, the susceptor could beprovided with lips or bumps in those areas to create passages with thering being flat or having feet. Similarly, while it is most practical tohave a space ring or spacer legs formed separately from the susceptor,similar structures could be formed integral with the susceptor.

Also, while the completely ring shaped blockers discussed above are thecurrently preferred shape, blocker that does not extend completely to aclosed 360° shape would be utilized. Similarly, a ring could be made astwo or more separate pieces that could substantially form a ring wouldbe useful. In addition a blocker not completely circular could be used.Other such changes are also included to come within the scope of theappended claims.

Although this invention has been described in terms of certain preferredembodiments, other embodiments are also within the scope of thisinvention. For example, although some of the illustrated embodiments aredescribed for specific sizes of wafers, the same features may also beused to accommodate larger wafers. Indeed, wafers of 300 mm or largerare presently contemplated to supplement traditional 200 mm and smallersized wafers. With larger wafers it may be desirable to employadditional spacers in a ring spaced radially inwardly from the threespacers 100 shown in FIG. 18, and offset circumferentially to be betweenthe spacers of FIG. 18.

What is claimed:
 1. An apparatus for processing a generally planarsubstrate comprising:a susceptor having an area for receiving saidsubstrate in a generally horizontal orientation; one or more spacersextending above the susceptor to support the substrate and form a gapbetween the substrate and the susceptor; one or more passages in saidsusceptor for introducing sweep gas into said gap to flow radiallyoutwardly from beneath the substrate; and a blocker ring supported onsaid susceptor at the periphery of said area to be beneath an outerannular portion of the substrate, the ring being configured to blockradial flow of sweep gas and block deposition gas from flowing into saidarea, wherein the ring is configured to create a thin annular purge gaspassage between the ring and the substrate.
 2. The apparatus of claim 1,wherein said ring has a thin generally rectangular cross-section.
 3. Theapparatus of claim 2, wherein said ring has an annular rib extendingupwardly from a main body portion of the ring.
 4. The apparatus of claim3, wherein said rib is approximately centered between inner and outerdiameters of the ring.
 5. The apparatus of claim 4, wherein the rib hasa generally flat upper surface with rounded corners.
 6. The apparatus ofclaim 5, wherein said rib flat upper surface is about 0.020 inches inradial dimension.
 7. The apparatus of claim 5, wherein said rib flatupper surface has a radial dimension which is about a third of theradial dimension of the rib.
 8. The apparatus of claim 2, wherein saidrib is located adjacent the inner diameter of the ring, thereby makingthe ring cross section approximately L-shaped.
 9. An apparatus forprocessing a generally planar substrate comprising:a susceptor having anarea for receiving said substrate in a generally horizontal orientation;one or more spacers extending above the susceptor to support thesubstrate and form a gap between the substrate and the susceptor; one ormore passages in said susceptor for introducing sweep gas into said gapto flow radially outwardly from beneath the substrate; and a blockerring supported on said susceptor at the periphery of said area to bebeneath an outer annular portion of the substrate, the ring beingconfigured to block radial flow of sweep gas and block deposition gasfrom flowing into said area, wherein said ring has a plurality ofcircumferentially spaced legs which create a plurality ofcircumferentially spaced passages between the ring and the susceptor,whereby restricted sweep gas flow is permitted above and below the ring.10. The apparatus of claim 9, wherein said gap is about 0.075 of an inchand said ring is about 0.065 of an inch in vertical dimension.
 11. Theapparatus of claim 9, wherein said susceptor has a shallow recess whichforms said substrate receiving area.
 12. An apparatus for processing agenerally planar substrate comprising:a susceptor having an area forreceiving said substrate in a generally horizontal orientation; one ormore spacers extending above the susceptor to support the substrate andform a gap between the substrate and the susceptor; one or more passagesin said susceptor for introducing sweep gas into said gap to flowradially outwardly from beneath the substrate; and a blocker ringsupported on said susceptor at the periphery of said area to be beneathan outer annular portion of the substrate, the ring being configured toblock radial flow of sweep gas and block deposition gas from flowinginto said area, wherein said spacers are integral with said ringextending upwardly from a main body portion of the ring to createcircumferentially spaced passages between the ring and the substrate.13. The apparatus of claim 12, wherein said ring includes a plurality ofcircumferentially spaced legs depending from a main body portion of thering, thereby creating a plurality of circumferentially spaced passagesbetween the ring and the susceptor.
 14. The apparatus of claim 13,wherein said spacers are circumferentially spaced from said legs so thata thermal path between the bottom surface of a leg and the top surfaceof an adjacent spacer is greater than the height of the ring includingthe spacer and the leg.
 15. An apparatus for processing a generallyplanar substrate comprising:a susceptor having an area for receivingsaid substrate in a generally horizontal orientation; one or morespacers extending above the susceptor to support the substrate and forma gap between the substrate and the susceptor; one or more passages insaid susceptor for introducing sweep gas into said gap to flow radiallyoutwardly from beneath the substrate; and a blocker ring supported onsaid susceptor at the periphery of said area to be beneath an outerannular portion of the substrate, the ring being configured to blockradial flow of sweep gas and block deposition gas from flowing into saidarea,wherein said susceptor includes a substantially disk-shaped lowersection and a substantially disk-shaped upper section, having a lowersurface in engagement with an upper surface of said lower section, saidone or more gas passages being defined by engaging surfaces of saidsections, one or more gas inlets in said lower section opening to itslower surface in said passages, and one or more gas outlets in saidupper section opening into said gap.
 16. The apparatus of claim 15,including a support for said susceptor, having a central shaft and aplurality of support arms extending radially and upwardly from saidshaft, with the arms having upper ends adapted to engage the lowersurface of said susceptor to support the susceptor, one or more of saidarms being tubular so that said sweep gas may be conducted through saidtubular arms into said inlets.
 17. An apparatus for processing asemiconductor wafer comprising:a horizontal susceptor having a recess inan upper surface; and a blocker slightly smaller than an inner diameterof the recess to fit within the recess between a wafer and the susceptorat the outer periphery of the wafer, wherein said blocker is configuredto create one or more gas passages between the susceptor and the waferthat permit sweep gas to flow radially outward from beneath the waferwhile blocking deposition gas from flowing radially beneath the wafer.18. The apparatus of claim 17, wherein said blocker is in the form of aring and has an annular rib extending above a main body portion.
 19. Theapparatus of claim 18, wherein said ring has a plurality ofcircumferentially spaced feet depending from the main body portion. 20.The apparatus of claim 18, wherein said rib is centrally positionedbetween inner and outer diameters of the ring.
 21. The apparatus ofclaim 18, wherein said rib is positioned adjacent an inner diameter ofthe ring.
 22. The apparatus of claim 17, wherein said blocker has aplurality of upwardly extending lands which create said gas passages.23. The apparatus of claim 22, wherein said blocker has a plurality ofcircumferentially spaced legs which create said gas passages.
 24. Theapparatus of claim 23, wherein said legs are circumferentially spacedfrom said lands.
 25. The apparatus of claim 22, wherein said blocker isconfigured to support said wafer.
 26. An apparatus for processing asemiconductor wafer comprising:a blocker to be positioned on a susceptorand beneath the periphery of a wafer, said blocker having an upwardlyextending rib, wherein the blocker has a plurality of circumferentiallyspaced feet depending from a main body portion.
 27. An apparatus forprocessing a semiconductor wafer comprising a ring to be positioned on asusceptor and beneath the periphery of a semiconductor wafer, said ringhaving a plurality of circumferentially spaced lands extending upwardlyfrom a main body portion.
 28. The apparatus of claim 27, wherein saidring has a plurality of circumferentially spaced feet depending from themain body portion.
 29. The apparatus of claim 28, wherein said feet arecircumferentially offset from said lands.